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The processes governing the (im)mobilization of Al, Fe and dissolved organic matter (DOM) in podzols are still subject to debate. In this study we investigated the mechanisms of (im)mobilization of Al, Fe and organic matter in the upper and lower B horizons of two podzols from the Netherlands that are in different stages of development. We equilibrated batches of soil material from each horizon with DOM solutions obtained from the Oh horizon of the corresponding soil profiles. We determined the amount of (im)mobilized Al, Fe and DOM after addition of Al and Fe at pH 4.0 and 4.5 and initial dissolved organic carbon (DOC) concentrations of 10 mg C litre−1 or 30 mg C litre−1, respectively. At the combination of pH and DOC concentrations most realistic for the field situation, organic matter was retained in all horizons, the most being retained in the lower B horizon of the well-developed soil and the least in the upper B horizon of the younger profile. Organic matter solubility seemed to be controlled mainly by precipitation as organo-metal complexes and/or by adsorption on freshly precipitated solid Al- and Fe-phases. In the lower B horizons, at pH 4.5, solubility of Al and Fe appeared to be controlled mainly by the equilibrium with secondary solid Al- and Fe-phases. In the upper B horizons, the solubility of Al was controlled by adsorption processes, while Fe still precipitated as inorganic complexes as well as organic complexes in spite of the prevailing more acidic pH. Combined with a previous study of eluvial horizons from the same profiles, the results confirm the important role of organic matter in the transport of Al and Fe to create illuvial B horizons initially and subsequently deepening and differentiating them into Bh and Bs horizons. [ABSTRACT FROM AUTHOR]

Interactions with dissolved organic matter (DOM) are generally believed to play a crucial role in the translocation of Al and Fe in acid sandy soils. Binding of Al and Fe to DOM affects their mobility in soils by altering sorption equilibria of charged sites on solid soil material, inducing precipitation of organo-metallic complexes and preventing the formation of inorganic Al and Fe phases. The relative importance of the different processes, especially with respect to the translocation of Al, Fe and organic matter in podzols, remains unresolved. We determined the effect of the presence of solid soil material from the eluvial (AhE and AE, respectively) horizons of a Fimic Anthrosol and a Haplic Podzol on the metal-to-organic carbon (M/C) ratio in solution and the formation of dissolved organic Al and Fe complexes. Furthermore, we assessed the resulting influence on the mobilization of Al, Fe and DOM. Even under considerable metal loading, the M/C ratios and ‘free’ metal fractions in solution remained low and relatively constant, due to an apparent buffering by the solid phase and the formation of organo-metal complexes in solution. The M/C ratios remained so low that significant precipitation of organo-metal complexes due to saturation with metals was not found. The apparent buffering by the solid phase can be explained by a strong release of organic matter from solid soil material and adsorption of non-complexed Al and Fe on solid organic matter upon metal addition. Adsorption of organo-metal complexes most likely played only a minor role. The observations confirm the expected mobilization of Al, Fe and DOM in eluvial horizons and seem to indicate that even under fluctuating input of Al, Fe and DOM the soil solution will have a constant composition with respect to M/C ratios and percentage of Al and Fe present in dissolved organo-metal complexes. [ABSTRACT FROM AUTHOR]